1. Field of the Invention
The present invention relates to a photodetector incorporating an avalanche photodiode and to an optical receiver incorporating the photodetector.
2. Description of the Background Art
Optical communication systems have been using an optical receiver incorporating an avalanche photodiode (hereinafter also referred to as APD). An APD has a function of amplifying a signal photocurrent, so that it is suitable as a photodetector to be used in an optical fiber communication system, which deals with a weak optical signal.
However, an APD has a characteristic in that its avalanche multiplication factor varies with temperature variation and the like. This characteristic is caused by the application of a relatively high reverse bias voltage as an applying voltage to the APD at the time of the avalanche multiplication. Consequently, the APD is forced to operate at a voltage in the vicinity of the breakdown voltage of the PN junction. As a result, its operating characteristic becomes highly sensitive to the variation in the ambient temperature and the like. Therefore, it is desirable to control the avalanche multiplication factor of the APD so that a constant multiplication factor can be achieved even when the temperature variation and the like occur.
The published Japanese patent application Tokukaishou 63-77171 has disclosed an optical receiver having a structure for controlling the multiplication factor of an APD. The structure of the optical receiver is shown in FIG. 13. The optical receiver has an optical-signal-receiving area 101 and an optical-signal-monitoring area 102 on a common substrate. These areas receive a signal lightwave from an optical fiber 103. An APD is formed in the optical-signal-receiving area 101. The multiplication factor at the optical-signal-receiving area 101 is controlled according to the monitor signal (output current value) from the optical-signal-monitoring area 102.
The optical receiver disclosed in the foregoing Tokukaishou 63-77171 has a structure in which a photodetection means for monitoring use such as the optical-signal-monitoring area 102 is provided on the same substrate that the APD is formed on. Therefore, the production of the optical receiver having such a structure requires the formation of two types of semiconductor photodetectors having different layer structures on the common substrate, rendering the production process complicated. As a result, an inevitable increase in the production steps and an accompanying reduction in the production yield incur an increase in the production cost.